This invention relates to a pulping process, and more particularly to a process for preparing a pulp from wood or other vegetable fibrous materials using a chlorine compound and ammonia which are recovered from the waste liquor in a subsequent stage.
Methods for preparing paper pulps may be broadly classified into one of three categories: (1) a mechanical method wherein pulp is produced by mechanically milling wood; (2) a chemical method wherein fibrous pulp is collected after treating the wood material with suitable chemicals to dissolve out the lignin which binds wood fibres together; and (3) a semi-chemical method wherein a gentle chemical treatment is employed in combination with a mechanical treatment. Both the chemical and semichemical methods involve a step for delignifying wood pieces which are generally called "chips" by the use of various types of cooking liquors. Such processes aim at making soluble the lignin which exists in the wood chips in a solid phase by the action of chemicals which exist in the cooking liquors in a liquid phase. Thus, the wood is delignified by a solid-liquid reaction. In this connection, it is necessary to uniformly infiltrate the cooking liquor into the chips so as to obtain a uniform and high quality pulp by a uniform delignification reaction. Moreover, in order to assure uniform infiltration of the cooking liquor into the chips, a predetermined infiltration period is required before the cooking liquor is heated to the cooking temperature at which the delignification reaction rapidly takes place. It is a common practice to provide a long infiltration period ranging from several ten minutes to several hours in industrial applications of a kraft process, a sulfite process or a semi-chemical process. Such long infiltration periods significantly lower productivity in the pulping process. It has been suggested that by using small size chips, disadvantages of such a long infiltration period can be effectively avoided. In this connection see, Nolan (TAPPI, Vol. 40 Page 170 (1957), and TAPPI, Vol. 51, Page 78 (1968), and Kleinert (TAPPI, Vol. 49, Page 53 (1963)) who experimentally confirmed the advantages of smaller size chips and proposed a rapid cooking process using fine chips. However, fibres obtained from fine chips generally suffer from mechanical damage to a greater degree, resulting in reduced pulp strengths. This has been confirmed by Hartler et al (Svensk Papperstidn 63 : 279 (1960)) through a number of investigations. Hence, it is not necessarily advantageous to employ too small a chip size for the purpose of shortening the infiltration period.
Various studies have been conducted on wood milling at elevated temperatures, including fundamental studies by Stone (TAPPI 38 : 449 and 452 (1955)), Lagergren (Svensk Papperstidn 60 : 632 and 664 (1957) and Pulp and Paper Mag. Can. October 1958, Page 217) etc. According to these studies, it has been found that (1) the energy required for milling wood is reduced when the wood is milled at high temperatures and (2) in milling wood at high temperatures, breakage of wood occurs mainly at the layers which exist between the fibres and which hold the fibres together, and therefore the breakage of the fibres per se is reduced. This is because the intermediate layers are mainly composed of lignin which becomes softer, easily yielding to breakage at higher temperatures. This phenomenon is industrially utilized, for example, in (1) the thermal crushing of wood in the production of wood fibre board, and (2) a thermal defibrating treatment in the production of high yield pulps which contain a large amount of lignin, such as a ground wood pulp, a chemi-mechanical pulp, a semi-chemical pulp, etc. Several types of industrial-sized equipment are actually in operation for the thermal defibration of fibrous plants. For example, Asplund's defibrator is well known in the art. The principle of the Asplund's defibrator and the minutes of its developing are set forth in a paper by Asplund (Svensk Papperstidn 56 : 550 (1953). Thus, it is well known that the thermal milling of vegetable fibrous material can be applied to the production of a starting material for wood fibre board and to a pulping process for obtaining a ground wood pulp, a chemi-mechanical pulp, a semi-chemical pulp, etc. However it has not been known that the treatment described above could be used as a pretreatment, i.e. refining of vegetable fibrous matter, in the preparation of the raw material for chemical pulp production. Rather, it has been generally believed that application of such a treatment would greatly reduce the strength of the resultant pulp.
The waste liquor resulting from the production of a chemical pulp has after been released into a river or sea as is. Such a practice results in a serious problem of environmental pollution due to the release of chlorine compounds, alkaline compounds, etc., which are contained in the waste liquor. Thus there exists a need to suitably treat and render the waste liquor harmless.
It is therefore an object of the invention to provide a method for the preparation of a starting material such as wood or other material for subsequent delignification by means of chlorine compounds and ammonia, and to recover useful chemical compounds from waste liquor to render the waste liquor innocous.
It is a further object of the invention to provide a method for recovering chlorine compounds and ammonia from waste liquor produced in the production of pulp.